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The field of microalgal-invertebrate symbiosis has gradually been undergoing some significant conceptual changes. The concept of low genetic diversity among symbiotic microalgae and high diversity among animal hosts has been challenged, and shown to be inaccurate. An increasingly high taxonomic diversity of microalgae are now recognized as forming symbioses with a high diversity of invertebrates, in marine and freshwater habitats, in benthic and pelagic communities. Information from a wide range of sources indicates that there is symbiont-host specificity in these symbioses at the species level, a realization that is completely in agreement with the patterns observed in other mutualistic and in parasitic symbioses. There has been a general misunderstanding that the less restrictive specificity observed under laboratory conditions is equivalent to that occurring in the natural environment. The mechanism of selectivity underlying the expressed specificity of microalgal-invertebrate symbioses is unknown, but evidence supporting the concept of molecular signalling between symbionts and hosts is rapidly accumulating. Such molecular ''cross-talk'' between symbionts and hosts has now been documented in bacteria-legume symbioses, and has also been implicated in interactions between plants and bacterial pathogens. Many aspects of the physiology, biochemistry and ecology of microalgal-invertebrate symbioses are better appreciated with the realization that functional diversity probably parallels genetic (taxonomic) diversity, and many of the apparently controversial aspects of the biology of microalgal-invertebrate symbioses can be reconciled by embracing this idea. Based on recent new evidence on the structure and composition of the photosynthetic apparatus of symbiotic dinoflagellates, it is apparent that some of the paradigms of functional, and ultimately, ecological aspects of the symbioses involving these algae will have to be modified. Of the various subdisciplines of the field, the most controversial, and indeed confusing, is that pertaining to the acquisition and utilization of nitrogen and phosphorus. I have attempted to review the various aspects of microalgal-invertebrate symbioses pertaining to their taxonomy, genetic diversity and evolution, morphology, biochemistry and physiology, as concisely as possible. As I believe that the field lacks an overall general theory that would guide future research directions, I conclude the article with a speculative discourse which hopefully may serve as a focal point. I believe that most of the ideas expressed, though not always supported by tangible evidence, are cast in the form of potentially testable hypotheses.